1. Field of the Invention
The invention relates to a method for producing Schottky diodes which have in the fringe area of the Schottky contact a doped self-aligned guard ring in the semiconductor layer underneath.
2. Description of the Prior Art
In the manufacture of Schottky diodes with very small areas, i.e. of Schottky diodes with a diameter in the order of 10 .mu.m, the elimination of the fringe effects presents considerable difficulties. Fringe effects cause, for instance, poor ideality, i.e. a strong deviation from the theoretical characteristic and in addition, a large cut-off current and a soft breakthrough. In large-area Schottky diodes, the undesirable fringe effects can be reduced substantially be means of a guard ring applied by means of a photomask. In Schottky diodes with a very small area however, a guard ring made by means of photomasks represents an additional capacitance so substantial that this technique cannot be used.
It has, therefore, been attempted to do without a guard ring in the manufacture of Schottky diodes with a very small area, and instead to reduce the undesirable fringe effects by means of elaborate purification methods. However, it has not been possible to produce, in a reproducible manner, Schottky diodes manufactured by such method for which reason such method is unusable.
At the conference "International Electron Device Meeting", Washington, D.C., Dec. 8-12, 1980, in a paper "A Very Small Schottky Barrier Diode (SBD) With Self-Aligned Guard Ring for VLSI Application" by Sang and Kim, consideration was given to diffuse-in a guard ring for Schottky diodes in a self-aligning manner. While the dimensions for guard rings can be reduced in small Schottky diodes by self-aligned masking, so that the additional capacitance due to the guard ring can be reduced to a tolerable level, the application of diffusion has disadvantages for small-area Schottky diodes made in this manner. The use of diffusion causes a limitation in the choice of the applicable Schottky metals and, because of out-diffusion during the diffusion process, causes a degradation of the series resistance of components. Because of the very low concentration of the doping substance in diffusing-in such guard rings, it is not possible to dope from the gaseous phase. Rather, the substantial extra cost of diffusion from the ampoule is necessary. In spite of this considerable extra cost, the reproducibility of Schottky diodes made accordingly is not assured sufficiently.